As is known generally, the usual process for producing carbon fibers is basically divided into a of flame-resisting step wherein acrylic fibers are subjected to heat treatment in an oxidizing atmosphere and a carbonization step where fibers from the flame-resisting treatment are subjected to heat treatment in an inert atmosphere. The step of imparting flame-resistance to acrylic fibers is practiced in an oxidizing atmosphere at temperatures of 200.degree. to 300.degree. C. over a period generally of 2 to 4 hours. This flame-resisting step requires 90% or more of the total time required by the process for producing carbon fibers. Accordingly it is apparent that reduction in carbon fiber production costs can be achieved by in shortening the time required for this flame-resisting reaction.
One of the methods for shortening the flame-resisting step is to raise the temperature of the treatment step as disclosed in Japanese Patent Publication No. 35938/72. However, since the flame-resisting reaction is exothermic as shown in Textile Res. J., 30, 882-896 (1960) this method when adopted may cause a vigorous uncontrollable reaction, which the inflammation of the acrylic fibers. Even if the inflammation is not induced, the acrylic fibers treated by this method will have flame-resisting structure on the peripheral portion of each filament, but will have insufficient flame-resisting structure in the inner portions thereof, thus turning into flame-resistant fibers of nonuniform flame-resisting structure. Such flame-resisting fibers in the later carbonization step will develop the undesirable phenomena known as napping and fiber break. This makes it difficult to effectively carbonize the fibers making it difficult to form high-performance carbon fibers. Japanese Patent Publication No. 25487/76 discloses a method which largely obviates these difficulties, and reduces the time for the flame-resisting treatment of the acrylic fibers to 5-30 minutes. This method comprises subjecting acrylic fibers to flame-resisting treatment under such conditions that the heat treatment time until the equilibrium moisture content of the acrylic fibers reaches 4% may be from 5 to 20 minutes, followed by carbonizing the fibers at a temperature of at least 1000.degree. C. However, the flame-resistance fibers having an equilibrium moisture content of 4%, as is evident from a number of literature references. These films exhibit an unsatisfactory flame-resistant structure and the cross section of each filament shown an outstanding double structure. Such flame-resisting fibers undergo pyrolysis in the later step of carbonization and micro-voids are formed in the resulting fibers. Hence it is difficult to convert these fibers into high-tenacity carbon fibers having a tensile strength of 400 kg/mm.sup.2 or more.
In this way, uncontrollable reactions occur during the flame resisting step and nonuniform flame-resisting reactions of acrylic fibers occur with the result that the fibers experience an increase in the number of acrylic monofilaments in a fiber a tow. Japanese Patent Application Laid-Open No. 163729/83 discloses an effective method for preparing A flame-resistance tow constructed of a large number of acrylic monofilaments. This method comprises heating acrylic tows, each constructed of 1000 to 30,000 filaments of 0.5 to 1.5 deniers in monofilament size in a flame-resisting oven at temperatures of 200.degree. to 260.degree. C. to convert the filaments into incompletely flame-resistant filaments having an oxygen content of 3 to 7%, thus preventing the filaments from fusing together during the later flame-resisting treatments of higher degrees) and treating the filaments under high-temperature flame-resisting conditions to convert the filaments into completely flame-resisting filaments having an oxygen content of at least 9.5%, followed by carbonizing the filaments. However, in this method, while napping or breaking of filaments does not occur, the conditions of the treatment for converting the incompletely flame-resistant filaments into completely flame-resistant filaments are harsh. Hence micro-voids are liable to develop in each filament. Moreover the oxygen content in the incompletely flame-resistant filaments is at least as high as 9.5%, and the filaments have a crosslinked structure caused by oxygen which develops therein to a high degree, so that it is impossible to effectively stretch the filaments to enhance the performance characteristics of carbon fibers obtained in the carbonization step and thus the tensile strength of the product carbon fibers is up to about 350 kg/mm.sup.2.
In recent years, carbon-fiber reinforced composites have been in extensive use for sporting, astronautical, industrial, and other applications and the expansion of their consumption has been remarkable. In response to such circumstances, significant effort is being made to improve the performance characteristics of carbon fibers.
With regard to the elastic modulus of such fibers, it was 20 ton/nm.sup.2 10 years ago, and improved to standard values of 23 to 24 ton/mm.sup.2 several years ago, and lately attempts have been made to prepare carbon fibers having an elastic modulus of about 30 ton/mm.sup.2. It is possible that such carbon fibers will be dominant in the future.
However, if improvement of the elastic modulus of carbon fibers is to be achieved while the strength of the fibers is kept constant, a decrease in the elongation of the fibers will be brought about, as a matter of course, and composites reinforced with such carbon fibers will be brittle.
Accordingly, there is an intense demand for carbon fibers of high elasticity and high elongation, that is, carbon fibers having high elongation and high strength at the same time.
The conventional method of improving the elastic modulus has been to raise the carbonization temperature, i.e. the temperature of the final heat treatment. This method, however, has the drawback that, as the elastic modulus is increased, the strength and consequently the elongation decrease. For instance, a carbonization temperature of about 1800.degree. C. is necessary in order to maintain an elastic modulus of 28 ton/mm.sup.2, but this temperature results in a strength at least 100 kg/mm.sup.2 lower than the value resulting from a carbonization temperature of 1300.degree. C.; thus a high strength cannot be achieved at all. Such a decrease in the strength with an increase in the carbonization temperature corresponds well with a decrease in the density. This is assumed to be caused by the development of micro-voids, which will bring about a decrease in the strength, in the fibers during elevation of the carbonization temperature. When acrylic tows each having a whole filament size of 1000 to 20,000 denier after flame-resisting treatment are subjected to a carbonizing treatment, it is also impossible to produce carbon fibers tows which have high strength and high elongation from such tows, that napping or filament breaking takes place frequently in the carbonization step. The causes of such deficiencies are exemplified by significant uneveness of the flame-resisting degree throughout the monofilaments which make up the tow, high unevenness in the longitudinal direction of each monofilament subjected to flame-resisting treatment, and minute flaws present in each monofilament itself subjected to a flame-resisting treatment.
As described above, presently that no technique is known by which acrylic fibers in tow form, each tow consisting of as great 1000 to 15,000 monofilaments, particularly a precursor consisting of such tows arranged in parallel in sheet form, can be subjected to a high-speed flame-resisting treatment for a period of up to 60 minutes and to a stretching treatment which enhances the performance characteristics of the carbon fibers in the subsequent carbonization step.
When fibers of high elasticity are produced, normally the carbonizing treatment is carried out at high temperatures, but it is extremely difficult using this method to obtain carbon fibers of high strength and high elongation. For example, such carbon fibers have the drawback that their tensile strength significantly varies. The fibers are obtained by subjecting flame-resistant fibers of 1.37 g/ml in density to treatment under tension in an inert atmosphere at a temperature of 200.degree. to 800.degree. C. and then heat-treating the resulting fibers in an inert atmosphere at a temperature of 1300.degree. to 1800.degree. C. According to the present study, it is believed that there is a problem in the inter-filament and filament lengthwise unevenness of flame-resisting degree. When the conventional flame-resisting method is employed, however, it is difficult to reduce the unevenness of the flame-resisting degree.
In one known method of imparting flame resistance to acrylic fibers, the treatment temperature is increased, which result in increasing the gradient of the temperature rise in the earlier stage of the flame-resisting step and decreasing the gradient of the temperature rise in the latter half of the step (see Japanese Patent Publication No. 35938/72). However, in this method fusion or agglutination occurs frequently among filaments. Further, a vigorous uncontrollable reaction occurs and inflammation is likely to result. In another method, the gradient of the temperature rise decreases in the earlier stage of the flame-resisting step and increases in the latter half of the step (see Japanese Patent Application Laid-Open No. 163729/83). In this method, the occurrence limited fusion or agglutination among the filaments occurs but the flame-resisting reaction proceeds rapidly in the latter half of the treatment, thereby increasing the interfilament and filament axis directional unevenness of the flame-resisting degree and frequently causing napping and filament breaking. In addition, the step passableness of this method is extremely inferior and it is difficult for this technique to provide high-performance carbon fibers.
With regard to carbonization a method is known in which fibers previously subjected to flame-resisting treatment are treated at a temperature of 250.degree. to 600.degree. C., then at a temperature of 400.degree. to 800.degree. C., and finally at a temperature of 800.degree. to 1300.degree. C. (see Japanese Patent Application Laid-Open No. 150116/84). However, carbon fibers having satisfactory performance characteristics are difficult to obtain by this method.
In the study leading to the present invention, the following has been discovered:
(i) In the prior art, the permeation of oxygen into acrylic monofilaments is retarded because of the inadequate rate of oxygen diffusion into the interstices between the monofilaments in tow form.
(ii) As a consequence, it is necessary that the density of the flame-resistant fibers which are to be fed to the carbonization step, should be increased to 1.40 g/ml or more, thus causing such undesirable properties as described above.
(iii) As a consequence, flame-resisting conditions are chosen so as to increase the rate of oxygen diffusion into tows of the acrylic fibers, which substantially improves upon the properties of the fibers and carbon fibers exhibiting extremely-high performance can be produced from the flame-resisting fibers obtained in this way.